Asphalt used for paving is a surprisingly complex material consisting of a multitude of different types of hydrocarbons. The relative proportion and chemical nature of these constituents depend on the crude source of the asphalt, and to some extent, on the refining method (i.e., whether the asphalt is obtained by distillation or by solvent extraction). It has been convenient to divide the asphalt into different fractions based upon the chemical reactivity. One such method, the Rostler method, divides asphalt into five fractions: asphaltenes; nitrogen bases; first acidiffins; second acidiffins; and paraffins. Another method, the clay gel method, divides the asphalt into asphaltenes, polar compounds, aromatics and saturates.
In turn, the asphalt is blended with a mineral aggregate which, when applied to the ground as an asphalt concrete in sufficient thickness, will harden and for a considerable period of time provide resistance to surface erosion by abrasion and dissolution, to compressive deformation, and to load fracture.
The performance of the asphalt concrete is closely related to the consistency of the asphalt cement used to glue the rocks together. The general measure of the consistency is its viscosity. As the pavement ages, asphalt oxidizes, which causes an increase in its viscosity. At some point the asphalt gets so hard that the asphalt concrete fails.
It is possible to reconstitute an aged asphalt by adding modifiers which replace materials that have been oxidized away, and redisperse certain fractions which have lost their solubility in the other parts of the asphalt. This reconstitution process is often done in situ by grinding the aged asphalt into a suitable range of sizes, and mixing the particles with a modifier which will revitalize the asphalt in the mixture. The rejuvenated mixture is then laid down using standard paving practice and, when it cools down, will function as well as a pavement made with neat asphalt. There are hot and cold processes by which to accomplish such recycling.
The hot process raises the temperature of the re-constituted mixture so as to provide an optimally flowable mixture which can be rolled to a suitable surface and which when cooled sets to a long-wearing pavement.
The cold process relies more on viscosity derived from the properties of the modifier and the lubricating effect of the water. When properly proportioned with a modifier, a suitable pavement can be provided. The cold process has the advantage of a lesser consumption of energy because of the absence or lesser use of heat.
Whatever the situation, the re-use of an aged asphalt to provide a new pavement requires removing the old pavement and processing it. Even in small projects this is a major tonnage operation, which involves either the use of old asphalt paving from a roadway to be recycled, or the use of an old pavement already removed and placed in a storage pit. In every case the process requires removal of the old pavement, then putting it back after it has been rejuvenated. In every situation, the subsurface grading must be attended to, and the recycled asphalt spread over it.
The disadvantages in rejuvenating old asphalt by picking it up, reconstituting it, and then laying it down have not gone unnoticed, and efforts to avert them quite reasonably lead one toward rejuvenating the asphalt pavement in situ if possible. By the phrase "in situ" is meant improving its properties by topical application of a modifier without moving the aged asphalt at all.
Ideally, a modifier for topical application would simply be absorbed into the aged asphalt to restore it to a homogeneous layer. In fact this does not occur. As the asphalt concrete ages, the asphalt in it hardens, and under cyclic loads from thermal sources as well as from traffic, it crazes and cracks. Gaps varying in size from hairline fractures to substantial separations occur throughout the entire body. The mere topical addition of a modifier cannot be expected to penetrate the entire asphalt body, nor to bring the pieces back into a continuous sheet. Conventional modifiers, when used with other means to fill major cracks, can extend the life of a pavement for a considerable of time. Still, the extent of penetration and the stabilization of the separated pavement pieces to resist cyclic loads, has been less than optimum.
It is an object of this invention to provide a modifier intended for topical application to an aged asphalt surface which readily enters cracks and gaps, which makes a substantial penetration into aged asphalt surfaces, and which provides bridge material to bond separated surfaces so they can suitably respond to and resist cyclic loads.
In fact, compositions according to this invention have under the most extreme and widely-varying applications, provided a topical modifier that is readily spread on aged asphalt, preferably but not necessarily brushed in, which after a few hours of rest for penetration and setting restores the aged pavement to general properties that approximate those which one would expect in a pavement only a few years old.